The White House is moving aggressively to limit Democratic defections on Capitol Hill that could undermine its negotiations with Iran, dispatching senior officials and President Barack Obama himself to lobby senators against taking action before a nuclear deal with the rogue regime is reached.

Senior administration officials have asked Senate Democrats to notify the White House if they are considering signing onto a bill drafted by Sens. Bob Corker (R-Tenn.) and Robert Menendez (D-N.J.) that would give Congress the ability to accept or reject any nuclear deal. The push, several Senate sources said, is to prevent a veto-proof majority from building by heading off any fresh Democratic support for the plan and persuade supporters to keep their powder dry until the conclusion of multilateral negotiations with Iran.

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The lobbying effort has come from all quarters. Obama has spoken directly with Democratic senators on the Foreign Relations Committee, including Ben Cardin of Maryland. Other senators who are weighing whether to join the legislative effort, such as Joe Manchin of West Virginia, have been briefed by the likes of Samantha Power, the U.S. ambassador to the United Nations. Vice President Joe Biden, Secretary of State John Kerry and Treasury Secretary Jack Lew have reached out directly to senators, according to sources on Capitol Hill.

And the White House chief of staff, Denis McDonough, warned lawmakers bluntly in a Saturday night letter not to interfere before a tentative deal is reached this month and the final terms are inked in late June.

All told, senators said, it amounted to a White House moving with dispatch to limit Democratic dissension in the face of a growing revolt from Congress.

“I would call it serious-minded,” said Maine Sen. Angus King, an independent who caucuses with Democrats and has discussed the Corker-Menendez bill with administration officials in recent days. He has co-sponsored the plan but has not said how he would vote if the measure heads to the floor imminently. “I think they are concerned.”

With their caucus divided, Senate Democratic leaders are letting the White House mount the lobbying push on its own. Asked Tuesday whether he was encouraging his members to oppose the Corker-Menendez bill, Senate Minority Leader Harry Reid said flatly, “No.”

With virtually all 54 Republicans expected to back the bill, and 11 Senate Democratic Caucus members signaling their support, the White House has little margin for error in heading off a veto-proof majority.

White House officials declined to comment. But they pointed to the McDonough letter, which showcased the administration’s thinking on Hill action as negotiations intensify between six world powers and Iran aimed at alleviating sanctions on the Middle East nation in exchange for curbing its nuclear program. The White House says there have been more than 100 meetings, hearings, briefings and calls with lawmakers on the matter since January.

In the letter to Corker on Saturday, McDonough reiterated Obama’s veto threats for the bipartisan legislation and urged Congress to hit the pause button until the final deal with Iran is reached. That could stretch into the summer after this month’s deadline to sign off on the outlines of a plan. McDonough said plainly that moving on Corker’s bill while any stage of the negotiations is ongoing would not serve “the country’s interests.”

“Let us complete the negotiations before the Congress acts on legislation,” McDonough wrote. “If we successfully negotiate a framework by the end of this month, and a final deal by the end of June, we expect a robust debate in Congress.”

Corker, chairman of the Foreign Relations Committee, is frustrated by the White House’s handling of the issue. He said Tuesday he remained “fairly” confident that his committee will vote on the Iran bill next week, coinciding with the March 24 deadline Democratic supporters of the bill have set before pushing forward. Detractors worry that a bad deal would only pave the way for Iran to build a nuclear bomb.

“They’re using every tactic available to try to keep Congress from playing its rightful role on Iran,” Corker said of the White House. “We’ve delayed the vote to accommodate people’s desire to let the 24th come and go. But I think we’re at a place now where we need to go ahead and move it through committee.”

That timeline would give Senate Majority Leader Mitch McConnell (R-Ky.) the ability to begin debate on Corker’s bill by perhaps mid-April, when the Senate reconvenes after Easter recess. But symbolic votes on Iran could take place on the floor next week when the Senate takes up its annual budget resolution.

While Menendez is a co-sponsor of the bill, he has said Congress should hold off acting until after the March 24 deadline. Whether he would wait until June is another question.

“As I said, we’re going to wait until the 24th, which was their deadline originally, and then we will come to a conclusion,” Menendez, the committee’s ranking member, said when asked about holding off until June.

Asked if he would commit to voting for the plan in the Foreign Relations Committee next week, Menendez would say only that he’s “talking to the chairman about it.”

But the White House whipping operation is causing some Democratic senators to have second thoughts about whether to support Corker’s bill in committee next week or in April, when Senate leaders hope to bring the bill to the Senate floor.

“What I got from the president is he needs the latitude to be able to negotiate, which I support. And this could impact his ability to negotiate,” said Cardin, who would not commit to voting for Corker’s bill even after March 24. “I want to give the president every opportunity, but I do think we have certain responsibilities in Congress.”

Sen. Bill Nelson (D-Fla.), who also co-sponsored the bill, said that it was “preferable” that Congress wait until after June to weigh in, but he wouldn’t say how he would vote before then.

“I’d prefer that they wait,” he said.

“I’m talking to the White House almost every day about this. I think their antenna is up,” said Sen. Chris Murphy (D-Conn.), a member of the Foreign Relations Committee. “My hope is their high level of activity this week is going to move the needle and persuade us to unite behind the president.”

But others are less concerned with the timing of congressional action, demanding instead that they get the bill ready so Capitol Hill has a vehicle for expressing its support or opposition to an Iran deal. The bill would also prohibit the president from suspending congressional sanctions on Iran for 60 days while Congress reviews the deal. And it would require that Obama report on Iran’s compliance with the deal every three months.

Democratic Sen. Tim Kaine of Virginia, another Foreign Relations Committee member, did not appear eager to heed the White House’s request to wait until June.

“Last Thursday, I talked to a key administration official who said, ‘I hope you’re going to wait at least until the end of March.’ Then it sounded like it’s kind of till the end of the June,” Kaine said. “Look, Congress is going to weigh in. … I don’t think the timing issue is that critical.”

King added: “I talked to [the White House]. I’m still listening. I thought the McDonough letter was well written and thoughtful, but right now I continue to believe having a congressional role is responsible.”

Others are also ready to buck the administration. Sen. Richard Blumenthal, a liberal Democrat, said he was asking for changes to the Corker-Menendez bill before he “probably” will sign onto it.

“Iran is at the table now because of sanctions that Congress has firmly pressed,” said Blumenthal, who has spoken to a “variety” of administration officials in recent weeks. “The principle of congressional review and the Senate’s role is an important principle that should be established from the outset.”

Still, the White House lobbying effort seems to have persuaded other Democrats not to break ranks. Sen. Jeanne Shaheen of New Hampshire, who sits on the Foreign Relations Committee, said she opposes moving forward on the matter in committee next week.

“The conversations that I had were about where they are in the negotiations, what currently the sticking points are, how they’re feeling about potential progress,” Shaheen said of her discussions with the administration. “I want to see if we get an agreement, what might be in that kind of agreement and then make a decision at that point.”